Ethylene vinyl alcohol copolymers (EVOH) show excellent oxygen and flavor barrier properties at low humidity, typically in the range of 0 to 60%. However, its barrier property deteriorates dramatically under high humidity conditions when the humidity is in the range of 75 to 90%. In fact, due to the polar nature of EVOH, such films made with EVOH generally exhibit poor moisture barrier. Therefore, EVOH is typically laminated with polyolefins on both sides to provide barrier properties for practical packaging applications in order to protect the EVOH from humidity effects. Moreover, EVOH is relatively brittle and difficult to stretch, tending to form cracks during stretching in biaxial orientation processes, for example, due to its crystalline nature. In biaxial orientation processes, EVOH grades that are suitable for stretching are typically limited to 48 mole % ethylene content. Lower ethylene content EVOH grades—which often exhibit better gas barrier properties—are unusable in orientation processes due to the brittle nature of these materials and will crack or fracture under the stretching forces involved. Another disadvantage of EVOH materials is that they require the use of costly adhesion promoters and/or tie-layer resins in order for them to bond adequately to polyolefin or polyester substrates. Without such tie resins, EVOH materials or related materials like polyviny alcohol (PVOH), tend to peel off easily from the polyolefin or polyester substrate resulting in loss of barrier properties and poor appearance. As a consequence of using these tie layer materials, the product cost of such a multilayer film or laminated increases due to these expensive resins and also due to capital expenditures to add multi-layer (3 or 4 or more) compositing dies for film orientation or extrusion coating.
U.S. Pat. No. 4,650,721 describes a process to improve the otherwise poor bonding of EVOH or PVOH in oriented films through the use of tie resins, namely maleic anhydride acid grafted polyolefins.
U.S. Pat. No. 5,153,074 teach a metallized oriented multilayer film design of EVOH and blends of a maleic anhydride modified propylene homopolymer or copolymer as the substrate to which the EVOH is contiguously adhered. The EVOH layer is used as a metallizing surface for the vapor deposition of aluminum. Again, the use of an adhesion promoting material is essential in this invention. It is known that EVOH is relatively hard to stretch compared to polypropylene. Consequently, only limited grades of EVOH like the one with 48 mole % of ethylene can be co-processed with OPP without forming any surface defects. Using lower ethylene mole % EVOH (e.g. 44% or 38%) in biaxial orientation causes surface defects like stress fractures or process issues like film breaks due to the higher crystallinity of the EVOH.
U.S. Pat. No. 5,175,054 teaches the solution coating of a mixture of solution-grade EVOH or PVOH containing about 80% of vinyl alcohol and aqueous dispersion-grade of the ionomer of the alkali salt of ethylene-methacrylic acid copolymer. This coating is applied to an oriented polymer substrate and subsequently metallized. In this invention, the ionomer acts as an adhesion promoter to assure adequate adhesion of the EVOH or PVOH to the polyolefin (polypropylene) substrate which is otherwise poor without the presence of the ionomer.
This invention seeks to avoid some of the disadvantages of EVOH containing laminate films.